I needed to do some research on this quickly - it was very unsettling to me... Here's some of what I found upon a short glance:
Chronic inflammatory bowel disease, cigarette smoking, and use of oral contraceptives: findings in a large cohort study of women of childbearing age
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1340036
Is the incidence higher because more women with IBD are using birth control to prevent pregnancy during a flare? Or is the incidence higher because the birth control causes the UC? This article doesn't say (that I saw).
Oral contraceptive use may increase bowel disease risk
http://www.patienthealthinternational.com/news/23063.aspx
"While there is strong evidence for the role of smoking and genetics, there has been a lack of convincing evidence for the link between exposure to oral contraceptive agents and the development of inflammatory bowel disease."